This invention relates generally to a high intensity discharge (HID) lamp ignition scheme, and more particularly to an ignition scheme for restarting an HID lamp.
An HID lamp generally refers to lamps of the high pressure mercury, high pressure sodium, metal halide, high pressure metal vapor and low pressure sodium types. An HID lamp is ignited through application of a high voltage pulse, nominally of several thousand volts, across the electrodes of the lamp. The high voltage pulses are produced by pulsed ignition circuits (i.e. ignitors). Many conventional ignitors operate continuously, that is, applying at least one ignition pulse across the lamp socket(s) every half cycle of the AC line voltage until the lamp lights. Continuous generation of high voltage pulses applied across the lamp socket(s) when no longer required (e.g. when the lamp has been previously removed from the socket(s) or has already failed (burned out)) subjects the ballast insulation and components to undue stress:
In order to control and thereby limit the time during which the high voltage pulses are generated by the ignitor, timer circuits within the ballast, such as disclosed in U.S. Pat. No. 5,070,279, have been employed. In such circuits (hereinafter referred to as shut-off type ignitors), the timer is set to activate the ignitor for a short, predetermined period after power is first applied to the lamp fixture and to thereafter disable operation of the ignitor until line power has been removed and then reapplied. Possible voltage stress on both the ballast insulation and components which might otherwise occur through continuous generation of ignition pulses is therefore reduced. This shut-off type ignition scheme, however, does not reignite the HID lamp when a voltage dip (transient) occurs sufficient to turn OFF the lamp but not low enough to reset the timer. The inability of a conventional shut-off type ignition scheme to attempt reignition of the lamp following extinction due to the aforementioned dip in line voltage is a serious drawback.
Shut-off type ignition schemes, such as disclosed in U.S. Pat. No. 5,070,279, also require the timer circuit to be powered based on the voltage applied to the lamp. No one timer circuit can be used for controlling ignitors which vary in ignition pulse amplitude and/or length of time during which the ignition pulses are generated since the timer circuit components must vary based on the nominally rated HID lamp voltage.
Accordingly, it is desirable to provide an HID ballast having a shut-off type ignition scheme which is sensitive to all dips (transients) in line voltage resulting in lamp arc extinction. The shut-off type ignition scheme also should be compatible for use with different types of HID lamps regardless of the nominally rated HID lamp voltage.